Design of inhibitors targeted to specific viral proteins become possible when the atomic structure of the proteins are determined by x-ray crystallography techniques. Today's fast computers and area detectors for data collections allow us to solve the structure of an inhibitor- enzyme complex in a few weeks time by use of difference Fourier techniques. The understanding of protein interactions with other functional groups also lets us design compounds with ligands targeted to a specific site on the protein surface. These data are stored in the structure data banks which are open for the public. In this application, we propose to design inhibitors targeted to type A influenza virus neuraminidase, human hepatitis A virus capsid and the attachment protein G and the RNA polymerase of respiratory syncytial virus. We have redetermined and refined the coordinates of two subtypes, N9 and N2, of type A neuraminidase and grown crystals of human hepatitis A virus. The inhibitor design will be based on the atomic coordinates, energetic calculations for the potential inhibitor-enzyme complexes and the knowledge of common interactions of functional groups observed in the known structures. The designed compounds will be examined by means of in vitro assays (such as enzyme activity assays and plaque assays) for their inhibitory effects. The will be further modified rationally based on the structural study of the inhibitor-enzyme complex by use of x-ray crystallography structure determination, interactive graphics, and energetic calculations. Our approach will be able to discover new reagents specifically targeted to the viral proteins and is carried on in rational steps, rather than random searches for new reagents.